Studies with multiple sclerosis (MS) have shown elevated antibody levels in CSF to measles virus, rubella virus and Epstein-Barr virus related antigens but not to Herpes simplex virus, cytomegalovirus or influenza type A virus. Abnormal induction of T-suppressor cells was found in response to measles virus in some MS patients during active disease. Normal activity of the natural killer (NK) cells was found but a lowered ability to respond to interferon was shown to occur in the NK cell cytotoxicity assay by lymphocytes from MS patients with active disease. Immune complexes were detected in CSF from active cases and highest levels were found in patients with acute exacerbation of disease. Abnormal immunoregulatory functions were demonstrated in MS cases and found to be temporarily associated with exacerbation of disease. JC virus (JCV) DNA has been detected in brain tumors produced in owl monkeys following inoculation with JCV. An enzyme linked immunosorbent assay has recently been developed to detect human antibody to JCV.